This invention relates to the use of nematodes for biological control of insect pests.
The infective larvae of Neoaplactana carpocapsae have been observed to destroy a wide range of insect pests under laboratory conditions. This ability is accounted for by the nematode's own pathogenicity towards the insects, and by its association with the symbiotic bacterium, Achromobacter nematophilus, which is usually to be found in the intestinal lumen of infective species. Following ingestion by an insect, the nematode will usually penetrate the gut wall to enter the haemocoel and release A. nematophilus which mutliply and lead to the death of the host by septicaemia. Additionally, there will often be reproduction of the nematodes in the dead host and hence generation of further infective organisms.
In the light of the above, many proposals for the use of infective larvae of N. carpocapsae in the biocontrol of insects, have been published since the nematodes were first investigated. A major obstacle to the implementation of such proposals has, however, been the susceptibility of nematodes to desiccation. Their apparent need for free water has led to suggestions that they be applied via aqueous media to bark or foliage on which the insect pests feed, but only limited effectiveness has been achieved with this technique under field conditions because of evaporation of the water before the nematodes locate and become established in the host. In order to reduce the rate of evaporation, it has been advocated that aqueous formulations should include, for example, wax evaporation-retardants, water thickeners, and gelling agents or surfactants, Unfortunately there is little evidence that such formulations so extend the life-time of nematodes in the field after application that they can be regarded as offering an effective and practical method of insect control.
It has been discovered that infective nematodes will remain active for substantial periods of time in the absence of free water, provided their body moisture, oxygen supply and mobility can be maintained, and that this can be achieved by use of oil instead of water as a medium in which to store or disperse the nematodes.
Many natural and synthetic oils possess appropriate oxygen permeability (i.e. high, relative to water), ability to reduce loss of body moisture, and mobility, to render them useful for this purpose, but some care is obviously necessary to avoid materials which although satisfactory in terms of physical properties, are toxic to the parasites. Optimum physical properties will vary according to factors such as the mode of application of the oil suspension and the conditions prevailing at the time of application. For most purposes a light mineral oil is recommended, and preferably a paraffinic oil containing no additives; we have found mineral oils with S.A.E. viscosity ratings in the range of about 50-250 to be satisfactory. One commercial example is DENTAX 140 oil produced by the Shell Company, another is TALPA 60, also produced by the Shell Company.
A preferred feature of such formulations is the addition of waxes and wax-like materials to the oil, to assist in reducing moisture loss from the nematodes. The beneficial effect of the wax is believed to derive from its tendency to migrate to the exterior of the oil droplet and form a coating which is substantially impermeable to water but not to oxygen. The wax is selected so as to have a melting point above the ambient temperature likely to be encountered during storage or in the field, but preferably it should be possible to mix it with the oil while molten in order to facilitate dispersion. Paraffin waxes with melting points of the order of 115-145.degree. F. are suitable when included in amounts from about 5 to 15% (by weight of oil). Although nematode survival (after spraying onto plants) is favoured by wax concentrations at the upper end of this range, a better kill is achieved with compositions containing less wax, about 6-8%, appears to be optimum; the latter compositions are also easier to spray. Although generally inferior to paraffin wax, acceptable alternatives include vaseline, petroleum jelly, wool wax, wool grease, microcrystalline wax and match wax.
A typical effective nematode medium has the following composition:
5-15 parts 135/145 paraffin wax PA1 95-85 parts TALPA 60 oil.
In addition, satisfactory performance is achieved when DENTAX 140 or TALPA 50 oils are used instead of TALPA 60, or 115/125 and 125/135 paraffin waxes are substituted in part or whole, for 135/145 paraffin wax.
It is difficult to say with precision how much oil medium is required to sustain a given number of nematodes; good results have been achieved with as many as 500,000/ml, but generally it is preferred to work with concentrations of the order of 200,000/ml.
When applied in an oil/wax formulation such as described above, the loss of moisture is so retarded that the nematodes take several hours to several days to desiccate, and even then they may be revived by moistening, as will occur upon ingestion by an insect. By contrast, nematodes subjected to the rapid drying, which is a feature of techniques employing aqueous formulations, die within a few minutes of the free water being lost.
Another discouragement to the use of nematodes as biological control agents for insects has hitherto arisen when attempts have been made to produce nematodes in quantity. Although small batches can be reared quite successfully in small vessels, such as petri dishes, containing growth media, when larger vessels are used the output is not found to be commensurate with the increased scale of operations. It is believed that this is because large uninterrupted masses of growth media are more easily contaminated by foreign bacteria, and also there are difficulties in aerating and harvesting nematodes from such masses of growth media. Such problems are minimised in a new method provided by the present invention.